1. Field of the Invention
The present invention relates to an optical control element, particularly, an optical control element in which a thin plate having a thickness of 10 μm or less and having an electro-optical effect is formed with an optical waveguide and a control electrode, and the thin plate is provided with a plurality of optical control portions.
2. Description of Related Art
In technical fields such as a long-distance optical fiber communication, a special optical measurement, or an optical control, an optical switch, particularly, an optical switch (an optical path switching and blocking type) using an electro-optical effect such as lithium niobate (LN) is used. As an optical control element such as an optical switch, an electronic waveguide type optical switch is widely used compared to a mechanical type switch. This is because the waveguide type optical switch has no movable part and is suitable for the increasing the speed of the switching (a switching speed is equal to or less than ns) or the integration. Furthermore, in the related art, many optical waveguide types are suggested which use the electro-optical (EO) effect such as lithium niobate (LN) or the like. In addition, a history of an optical switch using lithium niobate is specifically described in Optical Switching, Springer US, 2006, ISBN978-0-387-26141-6 Chapter 2 and Electro-optic Switch for PhotonicNetwork, H. Nakajima, Technical Report of IEICE, PS2002-15, 2005.
In the development of an ultra high speed optical switch using LN having the lowest dielectric constant and suitable for high speed operation among EO crystal materials, an internal reflection (Total Internal Reflection: TIR) type, a Y-Branch type, and an Asymmetric X-branch type optical switch have previously been suggested which are capable of performing a digital switching and are advantageous for miniaturization. Furthermore, a balanced bridge type, in which a size thereof is large in principle, is disclosed in Technical Report of IEICE vol. 109, No. 159, OPE2009-63, pp. 181-184, July, 2009.
For example, FIG. 1A is an internal reflection type switch in which crossed-optical waveguides 21 to 24 formed on a substrate 1 are switched by a control electrode 3 disposed in an intersection point 25. FIG. 1B is a balance bridge type switch in which portion 43 and 44 adjacent to two optical waveguides 41 and 42 are provided, a phase of an optical wave propagating through the optical waveguide is adjusted by an electric field to be applied to the electrodes 31 and 32, and an optical waveguide switching of lightwave in the adjacent portion 44 is controlled. In addition, in FIGS. 1A and 1B, the description of a ground electrode constituting the control electrode is omitted.
In addition, FIGS. 2A and 2B are Y-branch type switch, FIG. 2A is a plan view thereof, and FIG. 2B is a cross-sectional view in X-X′ of FIG. 2A. The substrate 1 is formed with ridge type optical waveguides 51 to 53 which constitute the Y-branch type optical waveguide. In the branched optical waveguides 52 and 53, signal electrodes 33 and 34 are formed via buffer layers 71 and 72. Furthermore, ground electrodes 61 and 62 are formed so as to surround the optical waveguides. By adjusting the voltage to be applied to the signal electrodes 33 and 34, in the optical wave incident from the optical waveguide 51, the intensity of the optical wave branched to the optical waveguides 52 and 53 is changed, thereby realizing the function of the optical switch.
The technical development of the optical switch is showing a tendency to stagnate as an overall development trend. The reason is because, when using an optical waveguide configuration or an electrode configuration formed of LN of related art, the driving voltage is too high to obtain a sufficient extinction ratio, whereby it is difficult to obtain the sufficient extinction ratio by an output voltage of a high speed driving circuit of a practical level. In addition, the expression “the extinction ratio is high” has the same meaning as the expression “the crosstalk is low”.
Furthermore, as a material having the EO effect higher than LN, there are KTN (potassium tantalate niobate), PLZT (lead lanthanum zirconate titanate), BT (barium titanate) or the like. However, since such materials have very high dielectric constant in the GHz band, in any electrode configuration, an operation at the GHz band or a speed higher than that is made difficult by the operation band limit due to the electrode capacity.
Meanwhile, the applicant disclosed a configuration for dramatically reducing the driving voltage of the LN optical modulator in international publication WO2007/114367. The invention is a technique that can realize speed matching between the microwave and the optical wave or impedance matching of the microwave and can greatly reduce the driving voltage in the optical modulator used in the LN substrate. The reduction in driving voltage has an advantage in that in the miniaturization of the optical waveguide element itself is possible, a low driving voltage type driving device of a low cost can be used, and the like.